Doctor of Philosophy / The drastic improvements in stiffness and strength even with the addition of small percentage of clay to a polymer are commonly traded-off with significant reductions in fracture toughness. It is believed that the presence of a stiff nano-filler will restrict the mobility of the surrounding matrix chains, and thus limit its ability to undergo plastic deformation, thereby decreasing their fracture toughness. To understand the role of rigid nano-fillers, like clay and their constraint effect on the surrounding polymer matrix, the effects of preferentially organized polyamide 6 lamellae in the vicinity of organoclay layers on the toughening processes are studied and compared with polyamide 6 filled with an elastomeric additive (POE-g-MA). It is suggested that to impart high toughness to polymer/organoclay nanocomposites, full debonding at the polymer-organoclay interface is necessary so that shear yielding of large volumes of matrix material can be enhanced. However, due to the strong tethering junctions between the individual organoclay layers and the matrix, full-scale debonding at the polymer-organoclay interface is rarely observed under stress conditions indicating that the constraint on the polymer adjacent to the clay is not relieved. Therefore, this has led to the development of ternary nanocomposites by adding a soft elastomeric dispersed phase to polymer/clay systems to obtain well-balanced mechanical properties. Polyamide 66/SEBS-g-MA/organoclay nanocomposites are prepared with four different blending protocols to understand the effect of blending protocol on the microstructure, mechanical properties and fracture mechanisms of the ternary nanocomposites so as to obtain new insights for producing better toughened polymer nanocomposites. In general, it is found that the level of enhancement of fracture toughness of ternary nanocomposites depends on: (i) the location and extent of dispersion of organoclay and (ii) the internal cavitation of rubber particles leading to effective relief of crack-tip tri-axial constraint and thus activating the matrix plastic deformation. Based on the wear/scratch damage studies on different polymer nanocomposite systems, it is suggested that elastic modulus and toughness of polymer nanocomposites are not the predominant factors controlling the material removal or friction coefficient and cannot be the sole indicators to compare and rank candidate materials. It is also found that nano-fillers by themselves, even if uniformly dispersed with good interfacial interaction with the matrix, do not irrevocably improve the wear (and friction) properties. Although it is important to consider these factors, it is necessary to thoroughly understand all microstructural parameters and their response to wear/scratch damage. Other important factors that should be considered are the formation of a uniform and stable transfer film on the counterface slider and the role of excessive organic surfactants or other modifiers added to disperse nanoparticles in a polymer matrix. It is also emphasized that the mechanisms of removal of materials during the wearing/scratching process should be studied meticulously with the use of high resolution microscopic and other analytical tools as this knowledge is critical to understand the surface integrity of polymer nanocomposites.

Doctor of Philosophy / The drastic improvements in stiffness and strength even with the addition of small percentage of clay to a polymer are commonly traded-off with significant reductions in fracture toughness. It is believed that the presence of a stiff nano-filler will restrict the mobility of the surrounding matrix chains, and thus limit its ability to undergo plastic deformation, thereby decreasing their fracture toughness. To understand the role of rigid nano-fillers, like clay and their constraint effect on the surrounding polymer matrix, the effects of preferentially organized polyamide 6 lamellae in the vicinity of organoclay layers on the toughening processes are studied and compared with polyamide 6 filled with an elastomeric additive (POE-g-MA). It is suggested that to impart high toughness to polymer/organoclay nanocomposites, full debonding at the polymer-organoclay interface is necessary so that shear yielding of large volumes of matrix material can be enhanced. However, due to the strong tethering junctions between the individual organoclay layers and the matrix, full-scale debonding at the polymer-organoclay interface is rarely observed under stress conditions indicating that the constraint on the polymer adjacent to the clay is not relieved. Therefore, this has led to the development of ternary nanocomposites by adding a soft elastomeric dispersed phase to polymer/clay systems to obtain well-balanced mechanical properties. Polyamide 66/SEBS-g-MA/organoclay nanocomposites are prepared with four different blending protocols to understand the effect of blending protocol on the microstructure, mechanical properties and fracture mechanisms of the ternary nanocomposites so as to obtain new insights for producing better toughened polymer nanocomposites. In general, it is found that the level of enhancement of fracture toughness of ternary nanocomposites depends on: (i) the location and extent of dispersion of organoclay and (ii) the internal cavitation of rubber particles leading to effective relief of crack-tip tri-axial constraint and thus activating the matrix plastic deformation. Based on the wear/scratch damage studies on different polymer nanocomposite systems, it is suggested that elastic modulus and toughness of polymer nanocomposites are not the predominant factors controlling the material removal or friction coefficient and cannot be the sole indicators to compare and rank candidate materials. It is also found that nano-fillers by themselves, even if uniformly dispersed with good interfacial interaction with the matrix, do not irrevocably improve the wear (and friction) properties. Although it is important to consider these factors, it is necessary to thoroughly understand all microstructural parameters and their response to wear/scratch damage. Other important factors that should be considered are the formation of a uniform and stable transfer film on the counterface slider and the role of excessive organic surfactants or other modifiers added to disperse nanoparticles in a polymer matrix. It is also emphasized that the mechanisms of removal of materials during the wearing/scratching process should be studied meticulously with the use of high resolution microscopic and other analytical tools as this knowledge is critical to understand the surface integrity of polymer nanocomposites.

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When many semicrystalline polymers are used as matrix materials in composites, a morphology known as the transcrystalline region is formed on the surface of the reinforcing material. This region introduces a new crystalline structure to the system that is different from that of the bulk matrix material. Whether this region is advantageous or detrimental to the mechanical performance of the composite has been debated. Therefore, efforts were made to control the size and structure of this region for a specific composite system, i.e., nylon 66 reinforced with high modulus carbon fibers. In many systems this additional phase can be avoided simply by altering the crystallization history of the matrix polymer. In this study, the interphase region is removed not by changing the crystallization history of the matrix, but by altering the crystallization kinetics of the matrix material by introducing a diluent which is known to induce such changes in blends of itself and the host polymer. The diluent in this study is poly(vinyl pyrrolidone) (PVP) which is a highly polar, uncrystallizable polymer, and the host polymer is nylon 66 which is a highly polar crystallizable polymer. Initially, microscopy studies were performed on blends of nylon and two molecular weights of PVP at very low diluent concentrations, i.e., < 7% by weight. Next, commercial high modulus carbon fibers were unsized by exposure to a benzene wash. In addition, sets of the unsized fibers were sized with various amounts and molecular weights of the diluent by exposure to dilute solutions. Both unsized and sized fibers were then embedded in the previously made blends, and an optical study of the morphological changes in the interphase is performed. Furthermore, preliminary studies of the fiber surfaces using x-ray photoelectron spectroscopy (XPS) were conducted. PVP dramatically reduced the nucleation density of spherulites and modified the lamellar organization in the spherulites (as evidenced by the occurrence of banding which is the twisting of lamella as they grow radially). Furthermore, in the presence of unsized fibers, the addition of small amounts of diluent to the matrix increased the size of the transcrystalline region. At slightly higher diluent concentrations, the nucleation density on the fiber surface was reduced. Only with sizing of the fibers with the diluent along with adding the diluent to the matrix was there a complete removal of the transcrystalline region. / Master of Science